There has been considerable interest in recent years in nonaqueous secondary cells because of the possibilities afforded both of obtaining energy densities considerably higher than those of the conventional and widely used lead acid storage batteries and of obtaining cells useful for small electronic applications, e.g., watches or calculators, that are superior to the generally used nickel-cadmium cells. Many materials have been considered as candidates for the electrode materials in nonaqueous cells.
One class of materials that has been the subject of much interest consists of the layered dichalcogenides of the transition metals of Groups IVB and VB of the periodic table. These materials are theoretically attractive electrode material candidates because a number of species, such as lithium or sodium, can move, i.e., intercalate, easily between the layers. A large number of ions of the intercalating species may often be incorporated in the layered dichalcogenide and high capacity cells result.
A large number of cells using transition metal dichalcogenides as active electrode materials have been investigated and several such cells appear promising as energy storage devices. Particularly promising are cells using TiS.sub.2 or Fe.sub.x V.sub.1-x S.sub.2, x greater than or equal to 0.2 and less than or equal to 0.5, as the active positive electrode material. These cells are described in Science 192, pp. 1126-1127, June 11, 1976 and Materials Research Bulletin 12, 825 (1977), respectively.
While these cells are promising, both in terms of weight and volume energy density and reversibility, improvement in some of their properties is desirable. In particular, increased capacity would be desirable. This could be accomplished by intercalating more than one ion per chalcogenide unit. However, attempts to intercalate more than one alkali metal ion per chalcogenide unit have been unsuccessful.
Additionally, the cells typically use negative electrodes containing free alkali metals, e.g., sodium or lithium. Use of such negative electrodes has several drawbacks. There are pratical problems involved both in assembling cells with free alkali metal electrodes because of both the reactivity of the alkali metal and the difficulty of controlling or minimizing dendrite growth during charging of the cell. Finally, a hazardous situation might be created if the cell structure ever became defective and free alkali metal became exposed to the external environment.